Refactor QCProperties

[tnatt]

Refactoring of QCProperties which includes:

• Improved readability of code
• Removal of old/unnecessary functionality
• Cleanup of tests
• Documentation updated with latest changes and user feedbacks Documentation: qcproperties #123 Updates for examples used in fmu-tools documentation #127

API changes

• Only properties of the same type (continous/discrete) allowed in a run. This avoids the confusion of having two dataframes whitin a QCProperties instance ref getting fraction per zone as a statistics for discrete parameter #111
• No longer allowed to specify "csvfile" in the input dictionary
• Statistics for discrete properties is now reported in fractions instead of percentages. Column names for discrete properties changed from "Percentage" and "Percentage_Weighted" to "Avg" and "Avg_Weighted" in alignment with the column names used for continous statistics.
• A Pandas dataframe is returned when running an instance method for extracting statistics instead of a PropStat instance

Added functionality

• A "range" option is added to filters to enable using continous properties as filters. It is also possible to add "range" as a value directly on a property to remove unwanted high/low values from the statistics.
• "COUNT" added as a clolumn to continous statistics to be able to see sample size adding the number of points in the statisitcs  #112

This PR closes #123 #112 #127 #111

[HansKallekleiv]

Looks good! Like the use of test classes 👍